Martian Poles (#2220)

* Let shader colorize the poles based on texture t coordinate.
* Apply soft edges for polar caps, with a slight undulation.
* Avoid polar caps on planet when rendered with survey (texture coordinates different in HiPS!)
* SUG: Add references

5 files changed, 71 insertions, 1 deletions

diff --git a/data/shaders/planet.frag b/data/shaders/planet.frag
index 6c39cba4f7..ed6f2c70d8 100644
--- a/data/shaders/planet.frag
+++ b/data/shaders/planet.frag
@@ -25,6 +25,7 @@ varying mediump vec2 texc; //texture coord
 varying highp vec3 P; //original vertex pos in model space

 

 uniform sampler2D tex;

+uniform mediump vec2 poleLat; //latitudes of pole caps, in terms of texture coordinate. x>0...north, y<1...south. 

 uniform mediump vec3 ambientLight;

 uniform mediump vec3 diffuseLight;

 uniform highp vec4 sunInfo;

@@ -298,7 +299,23 @@ void main()
     //litColor.xyz = clamp( litColor.xyz + vec3(outgas), 0.0, 1.0);

 

     lowp vec4 texColor = texture2D(tex, texc);

+

     mediump vec4 finalColor = texColor;

+	// apply (currently only Martian) pole caps. texc.t=0 at south pole, 1 at north pole. 

+	if (texc.t>poleLat.x-0.01+0.001*sin(texc.s*18.*M_PI)) {	// North pole near t=1

+		mediump float mixfactor=1.;

+		if (texc.t<poleLat.x+0.01+0.001*sin(texc.s*18.*M_PI))

+			mixfactor=(texc.t-poleLat.x+0.01-0.001*sin(texc.s*18.*M_PI))/0.02;

+		//finalColor.xyz=mix(vec3(1., 1., 1.), finalColor.xyz, 1.-mixfactor); 

+		finalColor.xyz=mix(vec3(1., 1., 1.), finalColor.xyz, smoothstep(0., 1., 1.-mixfactor)); 

+	}

+	if (texc.t<poleLat.y+0.01+0.001*sin(texc.s*18.*M_PI)) {	// South pole near texc.t~0

+		mediump float mixfactor=1.;

+		if (texc.t>poleLat.y-0.01+0.001*sin(texc.s*18.*M_PI))

+			mixfactor=(poleLat.y+0.01-texc.t-0.001*sin(texc.s*18.*M_PI))/0.02;

+		//finalColor.xyz=mix(vec3(1., 1., 1.), finalColor.xyz, 1.-mixfactor); 

+		finalColor.xyz=mix(vec3(1., 1., 1.), finalColor.xyz, smoothstep(0., 1., 1.-mixfactor)); 

+	}

 #ifdef IS_MOON

     if(final_illumination < 0.9999)

     {

diff --git a/guide/app_acknowledgements.tex b/guide/app_acknowledgements.tex
index 27da884b22..23960882d6 100644
--- a/guide/app_acknowledgements.tex
+++ b/guide/app_acknowledgements.tex
@@ -96,6 +96,7 @@ Stellarium would not look the same and work without these works from computer gr
 	\item[Nutation:]   \citetp{Nutation:IAU2000B} %Dennis D. McCarthy and Brian J. Luzum: An Abridged Model of the Precession-Nutation of the Celestial Pole.
 		This model provides accuracy better than 1 milli-arcsecond in the
 		time 1995-2050. It is applied for years -4000..+8000 only.
+        \item[Martian polar caps:] Based on data from \citet{MarsPoles:2009} and \citet{MarsRotation:2015}.\newFeature{0.22.0}
 \end{description}		
 		
 \section{Included Source Code}
diff --git a/guide/guide.bib b/guide/guide.bib
index b4e7d265ee..fe5a0c0b29 100644
--- a/guide/guide.bib
+++ b/guide/guide.bib
@@ -646,12 +646,34 @@ OPTnextEntry = {CD-ROM ISBN if this makes a difference},
 	  doi = {10.1007/s10569-019-9925-1}
 }
 
+@Article{MarsRotation:2015,
+  author = 	 {Sylvain Piqueux and Shane Byrne and Hugh H. Kieffer and Timothy N. Titus  and Candice J. Hansen},
+  title = 	 {{Enumeration of Mars years and seasons since the beginning of telescopic exploration}},
+  journal = 	 {Icarus},
+  year = 	 {2015},
+  OPTkey = 	 {},
+  number = 	 {251},
+  pages = 	 {332-338},
+  doi =          {10.1016/j.icarus.2014.12.014}
+}
+
+@Article{MarsPoles:2009,
+  author = 	 {David E. Smith and Maria T. Zuber and Mark H. Torrence and Peter J. Dunn and Gregory A. Neumann and Frank G. Lemoine and Susan K. Fricke},
+  title = 	 {{Time variations of Mars’ gravitational field and seasonal changes in the masses of the polar ice caps}},
+  journal = 	 {Journal of Geophysical Research},
+  year = 	 {2009},
+  OPTkey = 	 {},
+  volume = 	 {114},
+  number = 	 {E05002},
+  doi =          {10.1029/2008je003267}
+}
+
 @Article{Mallama:2007,
   author = 	 {Anthony Mallama},
   title = 	 {{The magnitude and albedo of Mars}},
   journal = 	 {Icarus},
   year = 	 {2007},
-  Ovolume = 	 {192},
+  number = 	 {192},
   pages = 	 {404-416},
   OPTannote = 	 {}
 }
diff --git a/src/core/modules/Planet.cpp b/src/core/modules/Planet.cpp
index 23b71848e4..698aee1f76 100644
--- a/src/core/modules/Planet.cpp
+++ b/src/core/modules/Planet.cpp
@@ -2768,6 +2768,7 @@ void Planet::PlanetShaderVars::initLocations(QOpenGLShaderProgram* p)
 	//common uniforms
 	GL(projectionMatrix = p->uniformLocation("projectionMatrix"));
 	GL(tex = p->uniformLocation("tex"));
+	GL(poleLat = p->uniformLocation("poleLat"));
 	GL(lightDirection = p->uniformLocation("lightDirection"));
 	GL(eyeDirection = p->uniformLocation("eyeDirection"));
 	GL(diffuseLight = p->uniformLocation("diffuseLight"));
@@ -3791,6 +3792,30 @@ void Planet::drawSphere(StelPainter* painter, float screenRd, bool drawOnlyRing)
 		}
 	}
 
+	if (englishName=="Mars")
+	{
+		// Compute Ls for Mars. From Piqueux et al., Icarus 251 (2015) 332-8 (9). Short algorithm with good approximation.
+		const double t=lastJDE-J2000;
+		const double M = (19.38095 + 0.524020769 * t)*M_PI_180;
+		const double sinM=sin(M);
+		const double sin2M=sin(2.*M);
+		const double sin3M=sin(3.*M);
+		const double Ls = 270.38859 + 0.524038542*t + 10.67848*sinM + 0.62077*sin2M + 0.05031*sin3M;
+		// Then compute latitudes of polar caps: Fig.10 in Smith, David E. et al. "Time Variations of
+		// Mars’ Gravitational Field and Seasonal Changes in the Masses of the Polar Ice Caps."
+		// Journal of Geophysical Research 114.E5 (2009): E05002. DOI:10.1029/2008je003267
+		//double latN= 70.-18.*sin((Ls-195.)*M_PI_180);
+		//double latS=-70.+18.*sin((Ls- 15.)*M_PI_180);
+		double latN= 70.+18.*cos((Ls-125.)*M_PI_180); // goes down to 52°
+		double latS=-68.+19.*cos((Ls-105.)*M_PI_180); // goes up to -49°
+
+		// Finally convert to texture coordinates.
+		float tNorth=static_cast<float>((latN+90.)/180.);
+		float tSouth=static_cast<float>((latS+90.)/180.);
+		GL(shader->setUniformValue(shaderVars->poleLat, tNorth, tSouth));
+	}
+	else
+		GL(shader->setUniformValue(shaderVars->poleLat, 1.1f, -0.1f)); // add some security margin.
 	GL(shader->setAttributeArray(shaderVars->vertex, static_cast<const GLfloat*>(projectedVertexArr.constData()), 3));
 	GL(shader->enableAttributeArray(shaderVars->vertex));
 	GL(shader->setAttributeArray(shaderVars->unprojectedVertex, static_cast<const GLfloat*>(model.vertexArr.constData()), 3));
@@ -3917,6 +3942,8 @@ void Planet::drawSurvey(StelCore* core, StelPainter* painter)
 		}
 	}
 
+	GL(shader->setUniformValue(shaderVars->poleLat, 1.1f, -0.1f)); // Avoid streaks across Mars
+
 	// Apply a rotation otherwize the hips surveys don't get rendered at the
 	// proper position.  Not sure why...
 	painter->getProjector()->getModelViewTransform()->combine(Mat4d::zrotation(M_PI * 0.5));
diff --git a/src/core/modules/Planet.hpp b/src/core/modules/Planet.hpp
index 863c671335..dc155bf01e 100644
--- a/src/core/modules/Planet.hpp
+++ b/src/core/modules/Planet.hpp
@@ -786,6 +786,9 @@ private:
 		int orenNayarParameters;
 		int outgasParameters;
 
+		// For Mars poles
+		int poleLat; // latitudes of edges of northern (x) and southern (y) polar cap [texture y, moving from 0 (S) to 1 (N)]. Only used for Mars, use [1, 0] for other objects.
+
 		// Moon-specific variables
 		int earthShadow;
 		int eclipsePush; // apparent brightness push for partial Lunar Eclipse (make bright rim overbright)